In connection with a surgical operation, it is often necessary to lower the patient's degree of consciousness to make it possible to carry out the operation. Therefore, the patient is put under anaesthetic. Anaesthesia in connection with surgical operations, however, is always combined with serious risks for the patient. Thus, the following serious, undesirable states may arise:
1. In spite of anaesthesia, the patient mentally experiences what is going on in the operating theatre. This can be experiences to be very unpleasant and can make the patient suffer mentally. PA1 2. The patient experiences pain but is not able to communicate with those around him. This state is very unpleasant and can make the patient suffer mentally. PA1 3. The patient dies owing to too high a degree of narcosis.
The states 1 and 2 occur owing to too low a degree of narcosis, whereas the state 3 is the result of over-dosing. Apart from the wish to prevent the above-mentioned serious consequences of incorrect dosing, there is a general wish to use as low a degree of narcosis as possible in order to shorten the time of awakening. This yields in a minimum effect on the patient and besides reduces the public nursing expense. Therefore there is a strong wish to be able to accurately determine the degree of narcosis, which is also called anaesthetic depth.
The monitoring of narcosis during surgical operation, however, is still a clinical problem. Modern surgery and the development of new anaesthetics have, if anything, accentuated this problem. Certainly the technical progress in the monitoring of narcosis has made it possible to follow vital parameters such as oxygen saturation, blood pressure, and the concentration of anaesthetic in the exhalation air. These possibilities, however, do not give a direct measure of the patient's anaesthetic depth, but they merely result in a rough picture of the patient's state. Moreover, there is a strong dependence on the type of anaesthetic used and the other drugs used in connection with the operation.
Progress in neurophysiological measuring methods has caused expectations of being able to continuously follow the patient's anaesthetic depth. In recent years, much research has been directed to the measuring of cerebral electric activities during anaesthesia and similar states with a reduced level of consciousness. A general example is the measuring of conventional EEG (electroencephalogram), which has not given any results. However, there have recently been reports on a so-called auditory evoked potential, i.e. an electric stimulus response to auditory stimulation, containing the necessary information. For comparison, it can be mentioned that EEG reflects the system at rest. Auditory evoked potential reflects the level of cerebral activities and is today used routinely for diagnosing hearing disorders by means of a brainstorm audiometer.
A brainstem audiometer registers the variation in voltage that occurs between two electrodes placed on the head when the brainstem and the auditory nerve are activated in acoustic stimulation. The response that is registered by the brainstem audiometer is part of an auditory electric response, AER, which is generated in the auditory chain, i.e. the auditory nerve and different parts of the brain, in acoustic stimulation.
The auditory electric response is usually divided in time as follows. Early responses (0-2 ms), quick responses (2-10 ms), medium-late responses (10-50 ms) and late or slow responses (50-300 ms).
In the range of early responses there is the summed-up action potential from the auditory nerve which is derived from the inner ear and the auditory nerve and which is registered by means of a technique called electrocochleography (EcoG). In the range of quick responses, there is the brainstem response that is generated in the auditory nerve and the brainstem and which consequently is registered by means of a technique called brainstem audiometry. In the range of slow responses, there is the auditory cortical response, which is substantially derived from the auditory part of the cerebral cortex and which is registered by means of a technique called cerebral cortex audiometry. The range of medium-late responses and the earlier part of the range of late or slow responses constitute the part of the auditory electric response that has been found to be the most interesting one to be studied for the purpose of determining the anaesthetic depth. In fact, this part of the auditory electric response is of different appearance at different anaesthetic depths. Besides, the changes are the same no matter what anaesthetic has been used.
For comparison, it can be mentioned that the early and quick responses, i.e. between 0 and 10 ms, change insignificantly during anaesthesia, and that the slow response changes, but its later part is also highly dependent on factors such as attention, sleep and sedatives, and therefore these responses are unreliable.
There is today a known method of measuring the anaesthetic depth by studying responses that are, above all, medium-late, but that can also be included in the range of late responses. The method is carried out by means of a modified brainstem audiometer. This brainstem audiometer is basically made up of three units, a registration unit comprising electrodes, amplifiers, filters and A/D converters; a stimulating unit comprising a signal generator, an amplifier and a sound generator; and a control unit/computer, to which the registration unit and the stimulating unit are connected. The control unit/computer coordinates the generation of sound and the collecting of data and processes the data values collected by means of the electrodes, and then finally presents the results.
The signals that one wants to measure are of the magnitude microvolt, while physiological noise signals from the rest of the brain, from the heart and the muscles can be of the magnitude hundreds of microvolt. Moreover, external noise signals may be present, in the form of electric or magnetic fields from the surrounding electric equipment or in the form of noise in the actual measuring equipment. The external noise signals can either be shielded or filtered off. The physiological noise signals are more difficult to handle. Thanks to the frequency band for the signals that one wants to measure being substantially known, the major part of the noise signals can be filtered off by means of said filters in the registration unit. However, there is a sufficient amount of noise signals in the frequency band for the signal that one wants to measure to make this disappear completely in the noise in the measured signal.
This problem is solved by means of averaging, which means that the signal one wants to measure is estimated by addition and means value calculation of a large number of individual, measured signals. These calculations take place in the control unit/computer. The addition of about one thousand measured signals is in many cases necessary in order to obtain an acceptable curve. By acceptable is meant that the above-mentioned change of the curve depending on the narcosis can be observed.
As mentioned above, the medium-late and late responses are taken into consideration. They are in the range of about 20-80 ms after the generation of the auditory stimulation. By applying the prior-art method for determining the narcosis, it thus takes, theoretically seen, at least 80 s to merely produce an acceptable curve, i.e. 1000 implementations each amounting to 80 ms. However, in practice the time is more than 2 min owing to the fact that the response must die away to a sufficient degree so as to prevent standing-wave ratios and interference phenomena from arising owing to overlapping responses. Certain circuit delays will also arise.
In continuous monitoring of narcosis in real time, quick responses are required. For this reason, one cannot rely on information that is several minutes of age. The above-described prior-art method and device for determining the anaesthetic depth therefore are not applicable in practice.